Moon being out illuminated hard stone pillar (8)
I believe the answer is:
monolith
'stone pillar' is the definition.
(I've seen this before)
'moon being out illuminated hard' is the wordplay.
'being out' indicates an anagram (out can mean wrong or inaccurate).
'illuminated' becomes 'lit' ('lit' can be a synonym of 'illuminated').
'hard' becomes 'h'.
'moon' anagrammed gives 'mono'.
'mono'+'lit'+'h'='MONOLITH'
(Other definitions for monolith that I've seen before include "Big pillar" , "Large corporation" , "Statue from one block of stone" , "Pillar-like stone" , "Large single upright block of stone" .)